Methods of making surface relief gratings

ABSTRACT

Photodetectors having low reflectivity triangular groove, surface relief gratings on homogenous material or one layer of a heterostructure. Preferably, the photodetector is a PIN photodiode in which the p-type layer is triangularly grooved. The surface relief gratings have an optical repeat distance greater than the wavelenth of light which impinges on the photodetector surface. Thus, zero order backward diffracted waves are not coupled into optical reflections which would thereby decrease the optical return loss (ORL). Furthermore, the surface relief gratings have minimum side-wall angles to limit reflection at the heterostructure interfaces from contributing to the ORL. The side-angles of the gratings are chosen to ensure that the angle of the higher order backward diffracted wave is greater than the capture angle of an optical receiver into which the photodetector is incorporated. The free-space depth of the grooves is a half-wavelength of the light impinging on the photodetector. Additionally, a novel ion milling technique is disclosed. This technique involves providing a rectangular groove grating initially etched by conventional chemical etching and photoresistive techniques and then ion milling at particular ion mill angles to obtain triangular groove surface relief gratings provided in accordance with this invention.

This is a division of application Ser. No. 07/346,182, filed May 1,1989.

FIELD OF THE INVENTION

This invention relates generally to optical systems which utilizephotodetectors. More particularly, this invention relates to triangulargrooves formed on the surface of homogeneous material, or on the surfaceof one layer of a heterostructure comprised of different layers ofmaterials having different refractive indices, to provide lowreflectivity surfaces on photodetectors utilized in optoelectronicmeasurement systems.

BACKGROUND OF THE INVENTION

Optoelectronic measurement systems are useful for determining variousoptical properties of optical systems, subsystems, components or samplesunder test. In general, light reflected from, transmitted through orgenerated by a unit under test (UUT) falls on a photodetecting devicewhich then transduces the light into an electrical signal. Thiselectrical signal is then analyzed to obtain various optical parametersassociated with the UUT. Myriad types of analyses can be accomplishedwith these optoelectronic measurement systems to yield highly usefulinformation about the UUT. In general, any type of light can be analyzedby optoelectronic measurement systems, for example, white light,monochromatic light, polarized light or unpolarized light.

Optoelectronic systems which make measurements of a UUT generallycomprise optical receivers that focus light from the UUT onto aphotodetector. One important optical receiver parameter is the "opticalreturn loss" (ORL) defined as:

    ORL=10 log (P.sub.i /P.sub.r);

where P_(r) is the reflected optical power from the receiver and P_(i)is the incident optical power to the receiver. The receiver ORL is thusa measure of the incident optical power to the receiver as compared tothe reflected optical power from the receiver.

It is critical that the ORL be maximized. For example, reflection fromthe receiver must be minimized to avoid perturbing other sensitiveelements of an optical system under test, such as a laser. Linewidthvariations from an indium gallium arsenide phosphide (InGaAsP)distributed feedback laser have been observed for reflected opticalfeedback as low as 0.000001%. See R. W. Tkach and A. R. Chraplyvy,"Linewidth Broadening and Mode Splitting Due to Weak Feedback in SingleFrequency 1.5 Micrometer Lasers," Elect. Lett. 21, 1081 (1985) and R. W.Tkach and A. R. Chraplyvy, "Regimes of Feedback Effects in 1.5Micrometer Distributed Feedback Lasers," J. Lightwave Technol., LT-4,1655 (1986). Thus, even relatively minor reflections from the receivercan perturb sensitive optical elements. Great effort has bee devoted inthe art to increasing the receiver ORL in optoelectronic measurementsystems to alleviate these problems.

A major cause of degraded receiver ORL occurs when light incident to thephotodetector utilized in the receiver reflects within the numericalaperture of the optical focusing means. One approach to increasing theORL in this situation is to coat the photodetector surface with anantireflection (AR) coating, thereby decreasing the level of reflectedlight from the photodetector. Single and multiple layer antireflectioncoatings have been used with homogeneous photodetector materials, suchas silicon or germanium. For example, it is known that for a homogeneousmaterial, a single antireflection coating having a refractive indexequal to the geometric mean of the refractive index of the homogeneousmaterial and the incident medium, and an optical thickness equal to aquarter wavelength of the incident light will cause the reflectedoptical power to be zero at the center wavelength.

However for planar InP/InGaAs/InP PIN heterojunction materialstructures, the implementation of an antireflection coating is limitedby the thickness accuracy of the organometallic vapor phase epitaxygrown InP and InGaAs layers. See, e.g., D. M. Braun, "Design of SingleLayer Antireflection Coatings for InP/In₀.53 Ga₀.47 As/InPPhotodetectors for the 1200-1600 nm Wavelength Range," Appl. Opt. 27,2006 (1988). Antireflection coatings disclosed in this articleexperimentally achieved 0.49% reflected optical power at a wavelength of1312 nm and 0.20% reflected optical power at a wavelength of 1515 nm.While AR coatings increase ORL, optoelectronic measurement systems withoptical elements having high sensitivities cannot tolerate an ORL ofthis magnitude. Thus, planar AR coated surfaces do not satisfy the needfor maximizing the ORL of receivers employing heterostructurephotodetectors in sensitive optoelectronic measurement systems.

Another approach to improving the OR of optical receivers is the use oflow reflectivity surface topographies. Known low reflectivity surfacetopographies in the form of grooves have typically been fabricated onhomogeneous, uniform materials. Grooved surfaces of this nature,referred to as "surface relief gratings," are disclosed in M. G. Moharamand T. K. Gaylord, "Diffraction Analysis of Dielectric Surface ReliefGratings," J. Opt. Soc. Am. 72, 1385 (1982); Erratum 73, 411 (1983); T.K. Gaylord, W. E. Baird and M. G. Moharam, "Zero Reflectivity HighSpatial-Frequency Rectangular-Groove Dielectric Surface ReliefGratings," Appl. Opt. 25, 4562 (1986); R. C. Enger and S. K. Case,"Optical Elements with Ultrahigh Spatial-Frequencies SurfaceCorrugations," Appl. Opt. 22, 3220 (1983). Furthermore, it is known thatdielectric surface relief gratings exhibit good antireflectiveproperties and very low diffracting efficiency in backward diffractedorders. See, e.g., T. K. Gaylord and M. G. Moharam, "Analysis andApplications of Optical Diffraction by Gratings," Pro. IEEE. 73, 894(1985).

However, the surface relief gratings disclosed in the aforementionedarticles require that the optical repeat distance of the grooves. Λ, beless than or equal to the wavelength of the incident light in freespace, λ. Under this condition, the ORL generally improves withincreased groove depth. The aforementioned Enger and Case articlesuggests that these high aspect ratio and high spatial frequency groovesact as a surface whose average index of refraction is smoothly taperedfrom that of air to that of the substrate material. Therefore, inheterostructure photodetectors having different layer refractive indicesand with the grooves formed in the top layer, the reflected power fromthe layer interfaces will efficiently couple to the zero order backwarddiffracted wave. These layer interface reflections can be significant.For example, the magnitude of reflection from a planar InP/InGaAsinterface for normal incident light with a wavelength of 1300 nm istheoretically calculated to be about 0.4%. Thus, these designsefficiently couple the large interface reflection to the zero orderbackward diffracted wave and are not conducive to limiting the ORL inoptical receivers employing heterostructure photodetectors used inoptoelectronic measurement systems. Deep grooves are also inappropriatefor small area PIN photodetectors since a very thick p layer is requiredto accommodate the grooves. The growth time for the p layer will causethe p and n type dopants to diffuse into the i layer causing performancedegradation. Therefore, known topographies with grooves having Λ ≦λcannot efficiently increase receiver ORL in photodetectors to achievelow reflected power.

SUMMARY OF THE INVENTION

The inventor of the subject matter herein disclosed and claimed hasdiscovered that low reflectivity triangular grooves in the form ofsurface relief gratings having Λ>λ, a free-space groove depth equal toan integral half-wavelength and a layer thickness one to three times thegroove depth achieve minimum reflections for normal incident light. Suchsurface relief gratings in heterostructures limit the contribution ofinterface reflections to the zero order backward diffracted wave. Thetriangular groove surface relief gratings are generally formed onphotodetectors comprising PIN photodiodes. Triangular grooves formed onthe surface of a photodetector in accordance with this invention alsomaintain a low level of the zero order backward diffracted wave forhomogeneous material.

Since Λ>λ for the surface relief gratings provided in accordance withthe present invention, higher order backward diffracted waves will bepresent. Thus, in preferred embodiments an upper limit on Λ is needed tomaintain the angle of the first order backward diffracted wave outsidethe capture angle of the optical receiving means in the optoelectronicmeasurement system. Preferably, by choosing the groove depth equal to ahalf-wavelength of the incident light to the photodetector and themaximum Λ, the multiple side-wall reflections within the groovesthemselves will be reduced, thereby reducing the ORL in optoelectronicmeasurement systems incorporating photodetectors provided in accordancewith this invention. Photodetectors having triangular grooves withuniform depth surface gratings provided in accordance with thisinvention experience less than 0.035% reflected optical power.

Accordingly, one embodiment of the present invention provides triangulargrooves formed on a surface to provide low reflectivity surface reliefgratings for a receiver in an optoelectronic measurement system. Infurther embodiments, the low reflectivity gratings comprise a pluralityof triangular grooves formed on a first layer, the grooves having auniform repeat distance greater than the wavelength of the lightincident on the first layer a uniform depth substantially equal to anintegral half-wavelength of the light detected by the first layer and aplurality of side-angles with respect to the first layer.

Additionally, methods provided in accordance with this invention makelow reflectivity surface relief gratings in the form of triangulargrooves on semiconductor materials. Preferably the grooves have asubstantially uniform repeat distance. In one embodiment, rectangulargrooves are formed on the semiconductor material surface by chemicaletching. The semiconductor materials are then ion milled in a lowpressure, ionizing plasma at a predetermined energy and current density.It is necessary to perform the ion milling step at two milling angles,and preferably for a time while the semiconductor material is elevatedwith respect to a material support platen to prevent the removedmaterial from redepositing on the grooved surface.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is an isometric view of a photodetector provided in accordancewith one embodiment of this invention having low reflectivity triangulargroove surface relief gratings.

FIG. 1B is an end view of the triangular groove surface relief gratingsof FIG. 1A.

FIG. 2 is an enlarged view of the triangular grooves of surface reliefgratings provided in accordance with this invention showing reflectionfrom an InP and InGaAs interface.

FIG. 3 is a block diagram of an optoelectronic measurement systemutilizing a photodetector with triangular groove surface relief gratingsprovided in accordance with this invention.

FIG. 4A is an enlarged end view of a rectangular groove surface etchedwith conventional etching and photoresistive techniques before novel ionmilling processes provided in accordance with this invention are appliedto the grating to obtain triangular grooves.

FIG. 4B is an illustration of novel ion milling processes provided inaccordance with this invention to produce triangular groove surfacerelief gratings.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Referring now to the drawings wherein like reference numerals refer tolike elements, FIG. 1A shows a heterostructure photodetector generallyindicated at 20. Preferably, photodetector 20 is a PIN photodiodecomprising three layers. A first layer 30 is a substrate or n-typelayer. A second layer 50 is interfaced at 40 with substrate 30 andcomprises an absorbing intrinsic or i layer. A third layer 60 isinterfaced at 70 with a second layer 50 and comprises a p-type layer.Triangular grooves 80 are formed on a surface 85 of third layer 60.

In one embodiment, the first layer 30 is a substrate of semiconductingmaterial, for example, indium phosphide (InP). For example, when thesemiconductor substrate 30 comprises indium phosphide, the substrate isan n+, silicon doped InP substrate which is preferably doped to anamount greater than 4×10¹⁸ cm⁻³. Second layer 50 is, for example, indiumgallium arsenide (InGaAs). For example, when InGaAs is used as secondlayer 50, it is generally desired to have the InGaAs layer undoped,i.e., having doping levels less than 4×10¹⁵ cm⁻³. Finally, the thirdlayer 60 generally comprises a semiconducting material, for example,InP, gallium arsenide, germanium or silicon, which in the embodimentshown is substantially thinner than layer 30. For example, when InP isused as third layer 60 it is generally desired to provide p⁺, zinc dopedindium phosphide, doped to a level greater than 1×10¹⁸ cm⁻³. Inaccordance with this invention, a triangular groove diffraction gratingis formed on the InP third layer 60 on surface 85 of the InP third layeropposite the InP/InGaAs interface 70.

Referring to FIG. 1B, triangular grooves 80 are formed on the InP thirdlayer 60 and have side-angles 90. In general, side-angles 90 can havedifferent values α and β. However, in one preferred embodiment α=β. Inone implementation, the side-angles 90 are about 20°.

Grooves 80 have a free-space depth, D, equal to an integral multiple ofan optical half-wavelength, n λ/2, wherein λ is the wavelength of thelight which falls on the photodetector and n is a positive integer. Inone preferred embodiment, n=1. For incident light at λ=1300 nm, thepower in the zero order backward diffracted wave is minimized at agroove depth of 650 nm (λ/2).

As shown in FIG. 1B, an antireflection coating 120, for example, siliconnitride, can be applied to grooves 80 to further reduce reflected powerin the zero order backward diffracted wave. AR coating 120 is providedto further reduce reflected power from the zero order backwarddiffracted wave at wavelengths when the groove depth, D, is not equal toan integral half-wavelength. AR coatings of this nature are designed tominimize reflections of unpolarized light with wavelengths of about 1550nm at normal incidence to photodetector 20.

Grooves 80 also have a uniform repeat distance, Λ, indicated generallyat 100. As herein defined and used throughout, Λ is the peak to peakdistance between each pair of adjacent grooves 80. Surface reliefgratings provided in accordance with the present invention arefabricated so that Λ>λ.

Referring to FIG. 2, a portion of the incident light 112 transmitted bythird layer 60 is reflected at interface 70 and then totally internallyreflected at the triangularly grooved surface 85, as shown at 110.Internal reflection 110 greatly improves the ORL for heterojunctionphotodetectors in optoelectronic measurement systems having surfacerelief gratings provided in accordance with the present invention. Forincreasing Λ, a larger portion of the light reflected from interface 70is totally internally reflected at 110. This is true in general until Λis so large that the side-angles 90 are so small that the lightreflected from interface 70 approaches surface 85 at near normalincidence and is no longer totally internally reflected.

Referring to FIG. 3, an exemplary optoelectronic system for measuringUUT 160 is shown. UUT 160 may be any type of sample which transmitslight and which must be analyzed for a particular application.Photodetector 20 has low reflectivity antireflective triangular groovesurface 85 wherein Λ>λ. Photodetector 20 is in communication with ananalyzer means 125. Analyzer means 125 is a typical standard instrument,such as an HP 8702A manufactured by the Hewlett-Packard Company, whichcompares the received electrical signal from photodetector 20 with theelectrical signal sent to a light source 140 which may be amonochromatic light source, for example, a laser, to determine the UUT'soptical parameters.

Light 150 from light source 140 falls on UUT 160. Light 150 may beunpolarized or polarized as is desired for the particular measurement.

Light 170 is transmitted through UUT 160 and is directed to opticalreceiving means 180. Optical receiving means 180 is operatively coupledto analyzer means 125. Optical receiving means 180 can compriseobjective lens 190 which functions to focus transmitted light 170 ontophotodetector 20. Objective lens 190 has a specific numerical aperture(NA).

Photodetector 20 has a low reflectivity surface provided in accordancewith this invention so that the ORL of photodetector 20 approaches theorder of 0.035%. In one preferred embodiment, optical receiver means 80comprises objective lens 190 and can further comprise photodetector 20as a single component in the optoelectronic measurement system. Opticalreceiving means 180 can further comprise an amplifier 183.

Because the repeat distance of the triangular grooves formed onphotodetector 20 is greater than the wavelength of the light from lightsource 140, higher order backward diffracted waves will exist in opticalreceiving means 180. The inventor has determined that to prevent higherorder backward diffracted waves from interfering with objective lens190, the angle of the first order wave must be greater than the captureangle of the objective lens 190 for rays that are incident at up to thecapture angle of the objective lens. This requires that:

    α>tan.sup.-1 (2NA),

for the case when α=β and D is an optical half-wavelength.

For example, for an objective lens 190 numerical aperture of 0.18, αmust be greater than 19.8 degrees. Therefore, higher order backwarddiffracted waves will not interfere with objective lens 190 and thus donot decrease the ORL of optical receiving means 180.

In accordance with this invention, the free-space groove depth of thetriangular groove grating is chosen to be equal to an opticalhalf-wavelength. With λ=1300 nm, a metric groove depth D=650 nm willminimize the power in the zero order backward diffracted wave.Therefore, surface relief grating designs provided in accordance withthis invention maximize ORL for high speed optical receiving means 180with normal incident light on photodetector 20. When λ is not equal to1300 nm and thus the groove depth is not an integral half-wavelength, ARcoating 120 reduces the power in the reflected zero order backwarddiffracted wave. Photodetectors provided in accordance with thisinvention can generally achieve ORL less than about 0.035%.

The present invention also provides methods of making triangular groove,low reflectivity surface relief gratings. In order to produce PINphotodetectors on semiconducting materials having low reflectivity,triangular groove surfaces, it is generally desired to first growepitaxial layers by organometallic vapor phase epitaxy (OMVPE) on n⁺,silicon doped InP substrates whose surfaces are oriented about 3° offthe (100) plane towards the (110) plane. After growing a 100 nm thickundoped InP buffer layer, an undoped, lattice matched InGaAs absorbinglayer about 1800 nm thick is grown on the InP substrate. Additionally, ap⁺, zinc doped InP layer of between 1750 nm and 3500 nm thick is grownon top of the InGaAs absorbing layer. In one preferred embodiment, theInP layer is 2100 nm thick.

In order to make triangular grooves, having side-angles of about 20°, anovel ion mill technique is applied to the semiconducting layers. Inaddition to producing symmetric, triangular grooves, this novel ionmilling technique can also fabricate asymmetric, triangular grooves. Itis generally desired to start the InP top layer at a thickness ofgreater than or equal to 1750 nm since the novel techniques provided inaccordance with this invention require about 1100 nm of InP to beremoved during the process while leaving about 650 nm (λ/2 of incidentlight).

In general, any type of conventional photoresistive and chemical etchingtechnique can first be employed to obtain a rectangular grating surfaceon the indium phosphide layer. In one preferred embodiment, the chemicaletching, photoresistive step consists of depositing about 50 nm ofsilicon nitride on the InP layer and using a photoresistive procedure toform about 0.75 micrometer wide photoresist lines in the (011)direction. A reactive ion etching step then etches field lines in thesilicon nitride. CF₄ can be used to accomplish the reactive ion etchingstep. It is then generally desired to remove the photoresist with anoxygen plasma and then wet chemical etch the field InP to a depth ofabout 750 nm in a 1:1 solution by volume of HCL/CH₃ COOH. After removingthe silicon nitride in a 5:1 solution by volume of NH₄ F/HF, the InPsurface generally has rectangular grooves.

Referring to FIG. 4A, the rectangular grooves obtained from theaforementioned conventional photoresistive, chemical etching techniqueare shown. An InP layer 60 about 700 nm thick has rectangular grooves210 etched thereon. After the conventional chemical etching technique isperformed to produce rectangular grooves 210, the novel ion milling stepprovided in accordance with the present invention is applied to obtain atriangularly grooved surface with minimum side-angles. The novel ionmilling technique is performed at milling angles α' and β', indicatedgenerally at 220 and 230, respectively. Angles α' and β', are preferablyin a range of about 15° to 45°, and more preferably in a range of about25° to 35° with respect to surface 210. For triangular groove formation,the inventor has determined that the optimum grating period in nm is:

    Λ=750+700(ctn α'+ctnβ').

In one preferred embodiment, the ion milling parameters are E=500 eV,J=0.45 mA/cm², P=1.5×10⁻⁴ Torr, and the plasma is comprised of a 70%argon (Ar) and 30% oxygen (O₂) mixture.

Referring to FIG. 4B, the ion milling step is illustrated. To preventremoved atoms from redepositing on the surface causing surfacemicrofeatures to form, photodetector 20 is held elevated above a platen240. In one preferred embodiment, photodetector 20 is held less thanabout 10 cm above platen 240 at the desired ion incident angles α' andβ'. In another preferred embodiment, photodetector 20 is held about 2 cmabove platen 240 at the desired ion incident angles. Photodetector 20 isheld elevated in this manner by photodetector holder 250.

Platen 240 is then rotated below the plasma Ar/O₂, field 260 as a DCbias is applied about the plasma by biasing means 270. Ion milling takesplace as the argon and oxygen ions are accelerated through the biasinggrid 280 and directed at photodetector 20. Biasing initiates ion millingof the indium and phosphorous atoms from the InP rectangular groovedlayer. It is generally desired to offset photodetector holder 250 fromthe center of platen 240 with photodetector 20 facing opposite thedirection of rotation of the platen.

During the ion milling step, the photodetector 20 is tilted alternatelyso that milling first takes place at angle α' and then at angle β'. Itis preferred that ion milling at α' and β' take place in three minuteincrements for a total time of approximately thirty minutes. After theion milling step is completed, the resulting surface profile is atriangular groove surface relief grating having side-angles α and βapproximately 10° less than the ion mill angles α' and β' and even morepreferably about 5 less than α' and β'. Furthermore, the final metricthickness of the InP top layer is about 1100 nm less than the startingthickness.

An AR coating 120 of silicon nitride can then be deposited on thetriangular grooves by plasma enhanced, chemical vapor depositiontechniques. Depending upon the desired resulting side-angles to beachieved with the methods herein disclosed, the AR coatings haverefractive indices of about 1.83 for silicon nitride and thicknessesvarying from about 200 nm to about 230 nm, most preferably about 210 nm.

Triangular groove, surface relief gratings for photodetectors fabricatedon InP top layers of InP/InGaAs/InP epitaxial structures by ion millingtechniques provided in accordance with this invention producephotodetectors having less than about 0.035% reflected optical powermeasured using monochromatic, unpolarized light of λ=1300 nm. Thereceiver ORL is minimized when the free-space groove depth of thetriangular groove surface relief gratings is about an integralhalf-wavelength of the incident light. When minimum side-angles α=β ≈20are formed, the application of an AR silicon nitride coating effectivelylowers the ORL for wavelengths of incident light where the grating depthis not equal to an integral half-wavelength. Antireflection coatingsapplied to grooves 80 aid in achieving similar results when the depth ofthe grooves is not equal to an integral half-wavelength of themonochromatic light.

Thus, optoelectronic measurement systems which utilize high speedoptical receiver means with objective lenses having numerical aperturesof about 0.18 have maximized ORL when photodetectors having triangulargroove, surface relief gratings provided in accordance with thisinvention are utilized. Therefore, triangular groove, surface reliefgratings and the methods of making these gratings satisfy long-feltneeds in the art for photodetectors having low reflectivity surfaces toprovide ORL of about 0.035% in optoelectronic measurement systems.

While described embodiments of photodetectors having surface reliefgratings provided in accordance with this invention are constructed onheterogeneous PIN photodiodes, homogeneous photodetectors havinggratings with Λ<λ and D=λ/2 would also maximize ORL in opticalreceivers. Furthermore, although exemplary materials and doping levelsare described for one implementation of a PIN photodetector inaccordance with the invention, materials and doping levels can bedifferent for other implementations utilized in other applicationshaving different PIN photodetector electrical conductivity requirements.Also, while PIN photodetectors are described, NIP photodetectors arealso contemplated with triangular grooves formed on the surface of then-type layer in accordance with this invention.

There have thus been described certain preferred embodiments of lowreflectivity, triangular groove surface relief ratings forphotodetectors. Additionally, methods of making these ratings have beendisclosed. While preferred embodiments have been described anddisclosed, it will be recognized by those with skill in the art thatmodifications are within the true spirit and scope of the invention. Thefollowing appended claims are intended to cover all such modifications.

What is claimed is:
 1. A method of making low reflectivity triangulargroove surface relief gratings on a material, the grooves having asubstantially uniform repeat distance, the steps of the methodcomprising:etching the material surface to form rectangular grooves onthe surface; and ion milling the material in a low pressure, ionizingplasma at a given energy and current density, the ion milling step beingperformed in a direction perpendicular to the rectangular grooves firstat a milling angle α' then at a milling angle β'.
 2. The method recitedin claim 1 wherein the milling angles α' and β' are in a range of 15° to45°.
 3. The method recited in claim 2 wherein the range of millingangles α' and β' is from 25° to 35°.
 4. The method recited in claim 3wherein the material is elevated to a height less than approximately 10centimeters above a platen to prevent removed material from redepositingon the surface.
 5. The method recited in claim 1 wherein the ion millingstep further comprises alternately tilting the material at the millingangles α' and β' in approximately three-minute increments.
 6. The methodrecited in claim 5 wherein the total time of the ion milling step isapproximately thirty minutes.
 7. The method recited in claim 6 whereinthe material is elevated approximately two centimeters above a platen.8. The method recited in claim 7, further comprising the step ofrotating the platen through an ionizing plasma.
 9. The method recited inclaim 8 wherein the material is offset from the center of the platen andthe material faces opposite a direction of rotation of the platen. 10.The method recited in claim 9 wherein the platen is rotated at a speedless than 10 rotations per minute.
 11. The method recited in claim 10wherein the platen is rotated at approximately four rotations perminute.
 12. The method recited in claim 11 wherein the ion milling stepforms triangular grooves having side-angles approximately 10° less thanthe milling angles α' and β'.
 13. The method recited in claim 12 whereinthe ion milling step forms triangular grooves having side-anglesapproximately 5° less than the milling angles α' and β'.
 14. The methodrecited in claim 13 wherein the triangular groove surface reliefgratings have an optimum uniform metric repeat distance, Λ, innanometers satisfying the following relationship:

    Λ=750+700 (ctn α'+ctn β40 )